For data transmission on a standard voice grade telephone line a modem is employed which is operative to modulate data in a form suitable for transmission along the telephone line, and to demodulate data received from the telephone line to reproduce the data transmitted from a remote site. The modem requires a power supply for energizing its electronic circuits, usually an AC power outlet or a battery source.
Providing the power for a modem is generally not a problem with computers which themselves operate from an AC power outlet. However, for battery operated portable computers, providing the power for a modem can be a significant drain on the limited power stored in the battery. This is particularly true if the modem is expected to monitor incoming calls over a long time period.
The ability to detect an incoming call and notify the host computer is a vital link to the useability of a particular modem/computer combination in providing electronic mail and remote database accessing functions. Any networking, database sharing or database updating system that relies on automatic dialing and answering between computers must have the ability to detect incoming calls, referred to as ring detection capability.
Modems in the past have had to have power on in order to detect and notify the host that an incoming call is being received. In a battery-powered computer, this is a critical waste of power which makes the system unacceptable. For example, if a computer consumed 1.3 mW in its "standby" mode while awaiting incoming data, but used a modem which required 225 mW of power to monitor and detect incoming calls, a battery which could provide one year "standby" power for the computer alone would provide only 40 hours of "standby" power for the computer and the modem together. Thus adding a high power consumption modem to a portable computer may negate the carefully designed power saving features incorporated in the computer.
There have been several alternative approaches proposed to provide ring detection and notification capability in the past, generally requiring that the modems be powered.
Modems commonly used with AC powered computers simply prohibit monitoring for incoming calls except at the times when full power is applied to the modem. An example of this approach is the Prentice Popcom X-100 modem. When power is disconnected, this type of modem cannot detect an incoming call or notify the host that such an event is occurring.
An alternative approach which provides ring detection with reduced power consumption is to allow parts of the modem's circuits to be powered off, keeping those circuits needed for ring detection running. An example of this approach is the Hewlett-Packard HP110 Portable Computer, which has a built in modem. It "shuts down" power to most of its modem circuitry to provide a lower-power ring detection capability, but still requires that some of its circuits have power on at all times.
Another proposed approach is to draw the operating power for the modem from the telephone lines rather than from the computer. Although this type of system does not deplete the computer's battery power, it suffers from other drawbacks. Federal regulations strictly limit the power that can be drawn from the telephone lines. This severely restricts the functions which a telephone line powered modem can perform. Typically, such modems are limited to low transmission rates in the range of 300 baud. Also, the power limitation prohibits implementation of the command set necessary to function as an "intelligent" modem.